1. Field of the Invention
This invention relates to an improved electrolytic capacitor and a method of forming the same. More particularly, this invention relates to an improved low voltage electrolytic capacitor having a barrier oxide layer with improved resistance to hydration formed by anodizing a valve metal, without prior formation of a thermal oxide layer thereon, in an aqueous phosphorus-containing organic acid electrolyte selected from the class consisting of monomeric phosphonic acid molecules, monomeric phosphinic acid molecules and mixtures of the same dissolved in an aqueous liquid. 2. Description of the Related Art
It is known to form electrolytic capacitors such as aluminum electrolytic capacitors by anodizing aluminum foil in a phosphoric acid electrolyte or a phosphate electrolyte, e.g., ammonium dihydrogen phosphate. For example, U.S. Pat. Nos. 4,164,779; 4,279,715; 4,427,506; 4,432,846; 4,479,167; and the English abstracts of Japanese Patent documents 62-134920 and 63-146424 all describe the use of phosphoric acid in the manufacture of aluminum electrolytic capacitors; while U.S. Pat. Nos. 4,113,579; 4,204,919; 4,470,885; 4,537,665; 4,580,194; and the English abstract of Japanese Patent document 63-146424 all describe the use of a phosphate such as for example, ammonium dihydrogen phosphate, in the formation of aluminum electrolytic capacitors. European Patent Application 246,825 describes an electrolytic solution for an aluminum electrolytic capacitor comprising a quaternary phosphonium salt. The English Abstract of European Patent Document 264,972 indicates that it teaches a method for cleaning aluminum surfaces by anodizing the aluminum in phosphoric acid to form surface oxide which is then dissolved as it forms.
It is also known to anodically form coatings on the surfaces of metals such as aluminum, using electrolytes including phosphonic acids, to enhance adhesive, bonding and/or spot welding to the metal surface; to enhance corrosion resistance, for example, for architectural application; and for lithographic applications. Such teachings may be typically found, for example, in U.S. Pat. Nos. 4,180,442; 4,381,126; 4,383,897; 4,399,021; 4,448,647; 4,788,176; 4,681,668; and European Patent Application 246,825.
U.S. Pat. No. 4,388,156 describes electrochemical treatment of aluminum substrates in a non-aqueous solution of a polybasic organic acid, such as sulfonic acids, phosphonic acids, phosphoric acids, or tribasic carboxylic acids, in an organic solvent, such as formamide, dimethylsulfoxide, aniline, dimethylformamide, mono-, di-, tri-ethanol amine, and tetrahydrofuran. The treated aluminum substrate is said to be provided with a surface which has improved adhesion to subsequently applied coatings which are useful for photographic elements in lithography or for capacitors and dielectric applications where a barrier layer is useful.
Conventionally, before such oxide dielectric layers are anodically formed on the valve metal surface, particularly in the formation of aluminum electrolytic capacitors, the aluminum metal is thermally oxidized by heating the metal to form a thermal oxide layer, and the subsequent anodically formed barrier oxide layer then forms beneath the thermal oxide layer. This two-step oxide formation process has been necessary to provide an oxide layer having the electrical properties needed to serve as the dielectric layer of the capacitor.
It would, however, be desirable to form an electrolytic capacitor by anodizing an electrolytic valve metal surface, such as an aluminum foil surface, using an electrolyte which forms a barrier oxide dielectric layer having increased resistance to hydration, and comparable capacitance values to prior art electrolytic capacitors, while eliminating the need for the prior art step of forming a thermal oxide layer on the valve metal surface before anodizing the metal surface.